This invention concerns a process for preparing glycolic acid. In particular, the process concerns the preparation of glycolic acid (hydroxyacetic acid) by the reaction of formaldehyde and formic acid catalyzed with liquid hydrogen fluoride.
The reaction of formaldehyde, carbon monoxide and water to produce hydroxyacetic acid is well known. For instance, U.S. Pat. Nos. 2,152,852; 2,153,064; and 2,265,945 disclose the use of acid catalysts, such as hydrochloric, sulfuric, and phosphoric acids, to promote the reactions under elevated temperature and pressure. Because the reaction requires relatively harsh conditions, attempts to improve the process have generally focused upon optimized concentrations of reactants and conditions or more effective catalysts. U.S. Pat. No. 3,911,003 granted Oct. 7, 1975 describes the preparation of hydroxyacetic acid by the hydrogen fluoride catalyzed reaction of formaldehyde and carbon monoxide in an aqueous environment. According to these patents the use of catalytic amounts of hydrogen fluoride provides very high yields of acid product under relatively mild reaction conditions.
Thus, the efficiency of the process for preparing acetic acid derivatives such as hydroxyacetic acid from formaldehyde, carbon monoxide, and water has been found to be surprisingly dependent upon the choice of catalyst.
It is also well known that formic acid, when heated in the process of concentrated sulfuric acid, decomposes into carbon monoxide and water. The mechanism of decomposition is described by Noller (Ed.) in the Chemistry of Organic Compounds, 3rd (1966) at pages 174 and 187:
"Because the carboxyl group is united to a hydrogen atom, rather than to a carbon atom as in all subsequent members of the series, formic acid undergoes a number of special reactions. When mixed with concentrated sulfuric acid, it decomposes into carbon monoxide and water." ##STR1## Thus, the decomposition of formic acid is caused by hydrogen ion attack on the negative oxygen of the formic acid molecule. Sulfuric acid, having a pK of 1.92 in aqueous solution readily provides hydrogen ion. Accordingly, the decomposition of formic acid is a special reaction catalyzed by a particular acid having a relatively high dissociation constant. For this reason formic acid is not generally accepted as a substitute for carbon monoxide and water in reactions catalyzed by relatively weak acids.
Nonetheless, formic acid is readily available and it would be an advantage to substitute formic acid for carbon monoxide and water in the process for preparing glycolic acid from formaldehyde, carbon monoxide, and water.